ObjectBinding should clear internal value when invalidated to avoid leak

Currently the ObjectBinding keeps the value until the next call to get() and if noone calls that for a longer time the memory can not be freed.

In our apps we sometimes have huge object-graphs in computed bindings and if we forget to explicitly call get() (although we currently don't need the new value) we eat up a large amount memory.

This is a clone of https://github.com/javafxports/openjdk-jfx/issues/109